1. Field of the Invention
This invention relates to off-shore drilling rigs and more particularly to a load equalizing and shock absorber system for a jack-up leg on a mobile off-shore drilling barge.
2. Description of the Prior Art
As is well known in the art of drilling and producing off-shore wells for oil and gas, one of the methods used is to provide a working platform that is jacked up above the surface of the water having three or more legs implanted on the sea floor. One method of jacking up the platform is by use of a rack and pinion jacking system. Generally the legs for such a unit consist of a tower using three or more chord members with lacing members between the chords. When these legs are subjected to the forces and moments produced by high waves, wind and current, the leg bends. As the leg bends, a differential vertical deflection occurs between chord members on the opposite sides of the leg. This phenomenon tends to reduce the loading in one chord member and increase the load in the opposite chord member. This also causes unequal loading of the jack pinions. When the sea load is light to moderate, these vertical deflections are relatively small and can be absorbed by resilient pads between the jacks and the point of support. When the sea load is extremely high, the deflections increase to the point that it is impractical to absorb them solely with resilient pads.
When offshore mobile jack-up drilling units, as described above, are moved onto or off of a location in a seaway, the legs are lowered or raised relative to the platform and the lower ends of the legs strike the seabed as the platform responds to the vertical motions of the waves. In some cases, this has caused severe damage to the legs and/or to the platform. One method that has been used to overcome this problem is to use resilient shock pads mounted between the jack frame and the point of support. When using this method, the wave height in which the unit can be safely moved onto or off of a location is dependent on the shock absorbing characteristics of the resilient shock pads. As the wave height increases, a point is quickly reached where it is impractical to provide shock pads that can absorb the energy produced by the downward motion of the platform and legs, as the legs strike a hard bottom.